fix #25678: return matters for generated functions

Just explicitely check for `CodeInfo` objects when doing the
interpolation of the generated parts. I initially tried a more
complicated version of this, which avoided doing this check if there was
more than one generated part, but that's probably not necessary here.

src/julia-syntax.scm

@@ -285,7 +285,14 @@
 (define (generated-part- x genpart)
   (cond ((or (atom? x) (quoted? x) (function-def? x)) x)
         ((if-generated? x)
-         (if genpart `($ ,(caddr x)) (cadddr x)))
+         (if genpart
+           ;; can't use an ssavalue here since (caddr x) could be a return statement
+           (let ((tmp (gensy)))
+             `($ (block (= ,tmp ,(caddr x))
+                        (if (call (core isa) ,tmp (core CodeInfo))
+                            (return ,tmp)
+                            ,tmp))))
+           (cadddr x)))
         (else (cons (car x)
                     (map (lambda (e) (generated-part- e genpart)) (cdr x))))))
 

test/syntax.jl

@@ -2787,3 +2787,10 @@ macro m_nospecialize_unnamed_hygiene()
 end
 
 @test @m_nospecialize_unnamed_hygiene()(1) === Any
+
+# issue 25678
+@generated f25678(x::T) where {T} = code_lowered(sin, Tuple{x})[]
+@test f25678(pi/6) === sin(pi/6)
+
+@generated g25678(x) = return :x
+@test g25678(7) === 7